Rethinking the peatland carbon cycle - identifying the role of mixotrophs in the biological carbon pump – MIXOPEAT

Mixotrophic protists are tiny organisms that can play an intriguing double role in food-webs, shifting between being producers (using sunlight to make carbohydrates by photosynthesis) or consumers (i.e. predators). They are increasingly recognized for their significant contribution to carbon (C) cycling in aquatic ecosystems because they can either respire CO2 as they digest their food (mostly bacteria), or ‘breath in’ the gas, transforming it, and storing the carbon in their own biomass. Shifts in this function (photosynthesis vs. predation) or the abundance of mixotrophs may thus influence the magnitude and direction of the carbon flux of ecosystems.

Despite their dominance in peatlands, the contribution of mixotrophs to C cycling has been almost entirely ignored. Peatlands are large repositories for terrestrial C, consisting up to 500 Gt of accumulated belowground C globally. Peatlands accumulate C when input through photosynthesis exceeds C losses through respiration. Any changes in either rate in response to climate change may thus modify the capacity of peatlands to sequester and store C. Mixotrophs can employ either strategy, thus their per-capita switching may drive the entire peatland C dynamic from C sink to C source, or vice versa.
We will identify which abiotic (e.g. temperature and precipitation) and biotic (e.g. plant diversity and prey density) drivers that trigger shifts in the fitness and function of mixotrophs. To achieve this, we will use experimental and observational experiments in lab (task 1) and field (task 2) conditions manipulating the abundance of mixotrophs and the climate, respectively. In addition, we plan to model the different processes and mechanisms identified in the different experiments and build predictive models of the effect of mixotrophs on peatland C dynamic under different climatic scenarios (task 3). Thus, this proposal tackles our knowledge gap regarding the ecological role of mixotrophs, testing how climate changes impact their function in peatlands, and the consequences for peatland C fluxes.

This project builds on a history of collaboration and combined expertise to address this key and overlooked question about the role of mixotrophs in peatland C dynamic. This research is important for two reasons. First, this is a step towards understanding a complex ecological system that makes up a significant percentage of terrestrial C storage, which may involve overlooked, tiny, but abundant organisms. Second, if the rising temperatures cause mixotrophic photosynthesis to decrease, peatlands could wind up fixing less carbon than they usually do each year, further reinforcing global warming.